There's not much solid and simple info on how to connect your Sega Master System to a modern HD TV (720p or 1080p), soooo why not mention a simple and solid way to do it.
All you need is the $30 LevelHike HD Link Converter and a USB charger (the micro USB, which is the one most non-Apple cell phones used for a while before USB Type C). This device converts the signal over to HDMI. That's it.
The results are silky smooth and you will melt into your chair and experience pure contentment.
The reason for my little adventure into converting the original hardware's signal over to HDMI was that it was getting difficult and expensive (and too heavy) to find a proper tube CRT to get the most out of AV cables. I gave in and bought an inexpensive 24" Vizio that had 1080p capabilities. The signal out of the LevelHike is probably 720p but, why not.
Another reason to skip a clunky tube TV is because the SMS doesn't broadcast in a typical aspect ratio. It "fits" a bit better on 16:9 anyway.
Boy o boy my cabinet is dusty. Had to move stuff around to fit this in, I promise I'll clean it.